PROJECT SUMMARY Most U.S. children fall short of the recommended 60 minutes of daily moderate-to-vigorous physical activity (MVPA). Our research and that of others show that this gap disproportionately affects children who are female and overweight/obese. Programs designed to increase children's physical literacy (PL), defined as the competence, confidence, and motivation to be physically active for life, hold particular promise for closing this gap and achieving equity. Furthermore, policy recommendations promote school- and home-based PA interventions, and evidence suggests that interventions incorporating both settings may have synergistic effects. However, no prior studies have tested the efficacy of a multilevel, PL-focused intervention. Our overall objective is fill this gap by designing and testing a program that will reach children both at school and at home to increase their PL and, in turn, total daily PA and MVPA. We will meet this objective through a partnership between Tufts University and the New York Road Runners. This partnership is a unique and significant strength given our organizations' collective dissemination experience and networks to scale PA programs nationwide. Our preliminary studies include a feasibility trial of the school component of the physical literacy program. This informs our proposed study in which we aim to further develop a family engagement component and subsequently pilot test the multilevel program: Supporting Physical Literacy at School and Home (SPLASH). The SPLASH home component will be informed by focus groups with n=80 parents in NYC (n=40) and Greater Boston, MA (n=40). We will then pilot the multilevel SPLASH program to evaluate implementation processes and outcomes. This will be accomplished in n=8 New York City (NYC) schools using a mixed- methods approach. The learnings from the R21 phase will further inform the final development and implementation of the SPLASH program during the R33. In the R33 phase, we will conduct a group randomized trial in MA to determine the impact of the full, multilevel SPLASH program on children's PL and total daily PA and MVPA. We propose to assess these outcomes in n=8 randomly selected schools (n=4 intervention, n=4 control, delayed intervention) using a pre-post study design with objective measurement (accelerometry) of PA and valid, reliable tools for PL assessment. We will further examine whether effects of the multilevel program differ by child sex or weight status and test whether changes in PL mediate changes in daily total PA volume or MVPA. This randomized and controlled study will provide empirical evidence about the efficacy of a multilevel PL-focused program on children's PL and PA. If the results demonstrate effectiveness, Tufts and New York Road Runners collectively have the dissemination experience and networks to scale the program nationally.